/*
  Forum: https://forum.arduino.cc/t/serial-input-ang-receive-int-question/1188479
  Wokwi: https://wokwi.com/projects/381293653101267969
*/

constexpr char LF = 10;  // ASCII for Line Feed
constexpr char CR = 13;  // ASCII for Carriage Return

char incomingByte;
long value;
boolean valid;

void setup()
{
  Serial.begin(57600);
  message();
}

void loop()
{
  if (Serial.available())
  {
    incomingByte = Serial.read();

    switch (incomingByte)
    {
      // digits are decoded and added to value
      case '0' ... '9':
        valid = true;
        value = value * 10 + incomingByte - '0';
        break;
     // In case of LF or CR and after in minimum one valid input 
     // value is printed. Then the input message is called
     // where also value and valid are reset   
      case LF:
      case CR:
        if (valid) {
          Serial.println(value);
          message();
        }
        break;
      // If any other characters are detected an error message is printed,
      // the Serial buffer is cleared and the input message is called   
      default:
        // all other characters lead to an error message
        Serial.print("Error: You used a non-digit character:\t");
        Serial.println(incomingByte);
        SerialClear();
        message();
        break;
    }
  }
}

void message(){
  Serial.println("Please input an integer value:");
  value = 0;       // Reset the sum
  valid = false;   // Reset validity
}

void SerialClear(){
  while (Serial.available()) {
    char garbage = Serial.read();
  }
}